The Science of Discworld

by Terry Pratchett

  According to the most recent dating methods, the Cambrian began 543 million years ago. The Burgess shale was deposited about 530-520 million years ago.

  In the words of Discworld’s God of Evolution: 'The purpose of the whole thing is to be the whole thing. '

  THIRTY-ONE

  GREAT LEAP SIDEWAYS

  RINCEWIND WAS IN HIS NEW OFFICE, filing rocks. He'd worked out quite a good system, based on size, shape, colour and twenty-seven other qualities including whether or not he felt that it was a friendly sort of rock.

  With careful attention to cross-referencing, he reckoned that dealing with just those rocks in this room would take him at least three quiet, blessed years.

  And he was therefore surprised to find himself picked up bodily and virtually carried towards the High Energy Magic building holding, in one hand, a hard square light grey rock and, in the other hand, a rock that appeared to be well disposed to people.

  'Is this yours?' roared Ridcully, stepping side to reveal the omniscope.

  The Luggage was now bobbing contently a few metres offshore.

  'Er ...' said Rincewind. 'Sort of mine.'

  'So how did it get there?’

  'Er ... it's probably looking for me,' said Rincewind. 'Sometimes it loses track.'

  'But that's another universe!' said the Dean.

  'Sorry.'

  'Can you call it back?'

  'Good heavens, no. If I could call it back, I'd send it away.'

  'Sapient pearwood is meta-magical and will track its owner absolutely anywhere in time and space,' said Ponder.

  'Yes, but not this bit!' said Ridcully.

  'I don't recall "not this bit" ever being recorded as a valid subset of "time and space", sir,' said Ponder. 'In fact, "not this bit" has never even been accepted as a valid part of any magical invocation, ever since the late Funnit the Foregetful tried to use it as a last-minute addition to his famously successful spell to destroy the entire tree he was sitting in.'

  'The Luggage may consist of a subset of at least n dimensions which may co-exist with any other set of >n dimensions,' said the Bursar.

  'Don't pay any attention, Stibbons,' said Ridcully wearily. 'He's been spouting this stuff ever since he tried to understand HEX's write-out. It's completely gibberish. What's “n”, then, old chap?'

  'Umpt,' said the Bursar.

  'Ah, imaginary numbers again,' said the Dean. 'That's the one he says should come between three and four.'

  'There isn't a number between three and four,' said Ridcully.

  'He imagines there is,' said the Dean.

  'Could we get inside the Luggage in order to physically go into the project universe?' said Ponder.

  'You could try,' said Rincewind. 'I personally would rather saw my own nose off.'

  'Ah. Really?'

  'But the thought occurs,' said Ridcully, 'that we can use it to bring things back. Eh?'

  Down under the warm water, the strange creature's stone structure collapsed for the umpteenth time.

  A week went past. On Tuesday a left-over snowball collided with the planet, causing considerable vexation to the wizards and destroying an entire species of net-weaving jellyfish of which the Senior Wrangler had professed great hopes. But at least the Luggage could be used to bring back any specimens stupid enough to swim into something sitting underwater with its lid open, and this included practically everything in the sea at the moment.

  Life in the round world seemed to possess a quality so prevalent that the wizards even discussed the idea that it was some conceptual element, which was perhaps trying to fill the gap left by the nonexistent deitygen.

  'However,' Ridcully announced, 'Bloodimindium is not a good name.'

  'Perhaps if we change the accent slightly,' said the Lecturer in Recent Runes. 'Blod-di-min-dium, do you think?'

  'They've certainly got a lot of it, whatever we call it,' said the Dean. 'It's not a world to let a complete catastrophe get it down.'

  Things turned up. Shellfish suddenly seemed very popular, A theory gaining ground was that the world itself was generating them in some sort of automatic way.

  'Obviously, if you have too many rabbits, you need to invent foxes,' said the Dean, at one of the regular meetings. 'If you've got fish, and you want phosphates, you need seabirds.'

  'That only works if you have narrativium,' said Ponder 'We've got no evidence, sir, that anything on the planet has any concept of causality. Things just live and die.'

  And then, on Thursday, the Senior Wrangler spotted a fish. A real, swimming fish.

  'There you are,' he said triumphantly. 'The seas are the natural home of life. Look at the land. It's just rubbish, quite frankly.'

  'But the sea's not getting anywhere,' said Ridcully. 'Look at those tentacled shellfish you were trying to educate yesterday. Even if you so much as made a sudden movement they just squirted ink at you and swam away.'

  'No, no, they were trying to communicate,' the Senior Wrangler insisted. 'Ink is a natural medium, after all. Don't you get the impression that everything is striving? Look at them. You can see them thinking, can't you?'

  There were a couple of the things in a tank behind him, peering out of their big spiral shells. The Senior Wrangler had the idea that they could be taught simple tasks, which they would then pass on to the other ammonites. They were turning out to be rather a disappointment. They might be good at thinking, ran the general view, but they were pants at actually doing anything about it.

  'That's because here's no point in being able to think if you haven't got much to think about,' said the Dean. 'Damn all to think about in the sea. Tide comes in, tide goes out, everything's damp, end of philosophical discourse.'

  'Now these are the chaps,' he went on, strolling along to another tank. The Luggage had been quite good as a collector, provided the specimens didn't appear to be threatening Rincewind.

  'Hmph,' sniffed the Senior Wrangler. 'Underwater woodlice.'

  'But there's a lot of them,' said the Dean. 'And they have legs. I've seen them on the seashore.'

  'By accident. And they haven't got anything to use as hands.'

  'Ah, well, I'm glad you've pointed that out ...' said the Dean, walking along to the next aquarium.

  It contained crabs.

  The Senior Wrangler had to admit that crabs looked a good contender for Highest Lifeform status. HEX had located some on the other side of the world that were moving along very well indeed, with small underwater cities guarded by carefully transplanted sea-anemones and what appeared to be shellfish farms. They had even invented a primitive form of warfare and had built statues, of sand and spit, apparently to famous crabs who had fallen in the struggle.

  The wizards went and had another look fifty thousand years later, after coffee. To the Dean's glee, population pressure had forced the crabs on to the land as well. The architecture hadn't improved, but there were now seaweed farms in the lagoons, and some apparently more stupid crabs had been enslaved for transport purposes and use in inter-clan campaigns. Several large rafts with crudely woven sails were moored in one lagoon, and swarming with crabs. It seemed that crabkind was planning a Great Leap Sideways..

  'Not quite there yet,' said Ridcully. 'But definitely very promising, Dean.'

  'You see, water's too easy,' said the Dean. 'Your food floats by, there's not much in the way of weather, there's nothing to kick against... mark my words, the land is the place for building a bit of backbone ...'

  There was a clatter from HEX, and the field of vision of the omniscope was pulled back rapidly until the world was just a marble floating in space.

  'Oh dear,' said the Archchancellor, pointing to a trail of gas, 'Incoming.'

  The wizards watched gloomily as a large part of one hemisphere became a cauldron of steam and fire.

  'Is this going to happen every time?' said the Dean, as the smoke died away and spread out across the seas.

  'I blame the over-large sun and all those planets,' said Ridcully.
<
br />   'And you fellows should have cleared out the snowballs. Sooner or later, they fall in.'

  'It'd just be nice for a species to make a go of things for five minutes without being frozen solid or broiled,' said the Senior Wrangler.

  'That's life,' said Ridcully.

  'But not for long,' said the Senior Wrangler.

  There was a whimper from behind them.

  Rincewind hung in the air, the outline of the virtually-there suit shimmering around him.

  'What's up with him?' said Ridcully.

  'Er ... I asked him to investigate the crab civilization, sir.'

  "The one the comet just landed on?'

  'Yes, sir. A billion tons of rock have just evaporated around him, sir.'

  'It couldn't have hurt him, though, could it?'

  'Probably made him jump, sir.'

  THIRTY-TWO

  DON’T LOOK UP

  THE WIZARDS HAVE BEEN CONVINCED all along that a planet is not really a good place to put living creatures. A nice, flat disc, with an attendant turtle who can deal with any inbound rocks before they can wreak havoc, makes much more sense.

  It looks increasingly as if they're right. The more we learn about the history of our planet, and the greater universe in which it resides, the more we have to admit that the wizards have a point. Not about the shape of our world, but about how dangerously exposed it is without a turtle. The universe is riddled with flying rocks and awash with radiation; most of it is either close to absolute zero or hot enough to make a hydrogen bomb seem like a nice, comfortable bonfire. Yet, somehow, life managed to gain a hold on at least one planet, and to retain that hold for four billion years — despite everything that the universe threw at it. (Often literally.) And despite every kind of nasty that the planet itself managed to concoct.

  There are two ways to interpret this.

  One is that life is incredibly fragile, and that Earth is one of the few places where the conditions necessary for life managed to hold together long enough for life to develop, diversify, and thrive. At any moment some disaster could undo all that good work and wipe the face of the planet clean of living creatures. The crab civilisation is fictitious, of course, but it's in our story to make two important points. First, that there has been plenty of time for lifeforms at least as intelligent as us to evolve on Earth; second, that if they had done, they could easily have left no trace of their existence. Oh, and third... that there are plenty of ways in which they could have come to a sticky end. So we've been incredibly lucky to avoid going the way of the crab civilisation. On millions of other apparently suitable worlds, life was not so lucky; it either never got started, or something wiped it out. Life is a rarity; Earth may be the only place in the entire universe where that fragile miracle happened.

  The other is that life is incredibly robust, and that the conditions on Earth are sufficient for life to arise, but by no means necessary. Just because things worked out in a particular way here, it would be a mistake to conclude that the same events must happen everywhere else. An important implication of evolution is that life automatically adapts itself to whichever environment happens to be available. Boiling water at the bottom of the ocean? Just what extremophile bacteria need. Two miles down in the rocks? Super — it's nice and warm down there, and there's plenty sulphur and iron to provide energy. Thank providence there's none of that poisonous oxygen; terrible stuff, violently reactive, immensely destructive. Nothing could survive in an oxygen atmosphere ...

  Both points of view have their advocates, and both have a certain amount going for them. Until we get to other worlds and find out what's there, there will be plenty of room for disagreement and debate. And, perhaps, a synthesis. Already both viewpoints agree that however life arose here, Earth was no Garden of Eden. Our planet is by no means the ideal habitat for life. In order for living creatures to survive, evolution has had to solve a lot of difficult problems, and adapt to hostile conditions.

  You may not realise just how hostile. But think of the common disasters: fires, hurricanes, tornadoes, earthquakes, volcanoes, tidal waves, floods, droughts ... too much rain and we're up to our necks in water; too little and our crops won't grow and we starve.

  But those are feeble compared to the big disasters.

  We tend to think of the history of Life on Earth as the smooth growth of a single great Evolutionary Tree. But that image is getting very long in the tooth. The history of life is more like a jungle than a tree, and most of the plants in the jungle were strangled, squashed, or suffocated before they took even the first step on the road to maturity. And however that jungle grew, there was nothing smooth about it.

  True, there was a very long time when there were only 'blobs' in the seas, and we might think of that period as a fairly featureless trunk of the Tree. As far as the blobs were concerned, life probably was pretty uneventful — but only because they didn't notice what was happening to the planet. They were largely unaffected by a whole succession of events that would have been cosmic catastrophes for later, more complex life.

  There were certainly a few pretty big impacts at the beginning of life on the planet that didn't put them out of business, such as it was. And Snowball Earth — if in fact it ever happened — can't have been easy. But despite all these obstacles, or even because of them, life slowly changed, evolved and diversified as the eukaryotes learned to live in an oxygen atmosphere.

  That should have been a disaster. The very composition of the atmosphere changed, and all the biochemical tricks that had evolved to suit the available range of gases became obsolete. Worse, the gas polluting the air was oxygen, an appallingly reactive substance. Think of what would happen today if the atmosphere started to be taken over by fluorine. Some of the nastiest, most explosive substances are fluorine compounds. But oxygen is just as bad, if not worse; think of fires, think of rust, think of decay.

  The eukaryote cell triumphed over oxygen, and subverted it. Oxygen's negative characteristics were turned into positive ones. So effective was this evolutionary revolution that the deadly, poisonous pollutant became essential for (most) life. Deprive a human, a dog, or a fish of its oxygen, and it dies very quickly. Water, food ... those it can do without, for a time. But oxygen? You'll survive for a few minutes at most, maybe half an hour if you're a whale.

  The oxygen trick was so good that it took over. Eukaryote life radiated — diversified rapidly — in the seas, inventing entire new kinds of ecologies. With that diversification as a springboard, life came out on land. The advantage of moving to the land was that it opened up an entire range of new habitats, new ways of making a living. So many new kinds of living organism could thrive. One disadvantage, though, was that living on land made life much more vulnerable to astronomical insults. Living on land produced many more complicated kinds of plants and animals, able to protect themselves against small local changes, like hot sunshine, or snow. But, ironically, that very complication made them much more vulnerable to big problems — like stones falling from the sky.

  We all know about the meteorite that killed the dinosaurs ... and that fits. Dinosaurs were wonderfully effective as long as the environment remained suitable, but they were not at all well-adapted to the sudden changes that the impact created. But bacteria hardly noticed. If anything, it was a good time for them: they got a lot of extra food for a few hundred years, as the corpses decayed, and then went back to the old boring routine.

  We'll say a bit more about the 200 million year reign of the dinosaurs and their friends soon, and indeed about what killed them off. But we need to give you some context first. Simple forms of life can put up with a lot, and did. And they changed the planet, or at least its outer skin, by putting in feedback loops that made it less liable to change.

  They started Gaia. This is the name that James Lovelock gave in 1982 to the concept of the Earth as a complex living system — metaphorically, an organism in its own right. The idea has been romanticised into the Earth being a kind of Earth-mother, but what do
you expect when you attach the name of a goddess to your new scientific concept? Stripped of the romantic frills, the point is that our planet acts as a single system, and it has evolved mechanisms that keep it functioning effectively. This development is a consequence of innumerable subsystems — organisms, ecologies — evolving mechanisms that keep them functioning effectively. If every member of a team gets better at playing their role within the team, then the team as a whole improves too.

  Complexity is a double-edged sword. More complex forms of life find that the ordinary problems of living on a planet are more and more under control ... except for those confounded problems from outside, like meteorites, which can be disastrous.

  The Moon, Mercury, Mars, and various satellites are covered in circular craters, some large, some small. Nearly all of those craters, we now know, result from the impact of a big lump of rock or ice or a bit of both. A few are volcanic. Not so long ago most were thought to be caused by volcanoes, but that turned out to be wrong.

  Several planets, among them the Earth, do not show obvious signs of impacts. Is that because nothing hit them? No. An atmosphere helps: smaller bodies burn up before they hit the ground. It's the closest to a protector-turtle that we get. But bigger rocks can still get through the defences. The main reason why some planets show no clear signs of impacts is because those planets have weather, like the Earth, which erodes the craters until they disappear, or episodes of massive vulcanism, like Venus, which resurfaces the planet, or are gas to begin with, like Jupiter and Saturn, and don't show permanent marks.

  In Quebec there is a lake called Manicouagan. You can't miss it on a map: look near 51°N, 68°W. It's circular, and it's big: 44 miles (71 km) in diameter. It is the weathered remains of a gigantic crater that formed 210 million years ago when a rock two or three miles (3-5km) across collided with the Earth. There is a central peak made from rock that melted in the heat that the impact generated and then solidified; more molten rock flowed across the floor of the crater and still can be found today. The lake fills a ring-shaped valley that glaciers carved out of soft rock that originally formed the crater walls, and was eroded away and collapsed.